If the CH is on and the system up to normal running temperature and pressure (about 1.7bar) I turn on the hot tap and the boiler switches from CH to HW and I get HW no problem.
I then turn off the hot tap and the CH does not come back on, if I again turn on the hot tap I get HW until the HW system is drained and then the cold comes through. No clicks, no hot water light on the front, but I can hear the water flowing through the boiler.
I have to wait for the system to cool down and drop pressure for the CH to come back on and for the HW to work. All seems fine with the HW if the system is cold.
I've recently had the 2 diaphragms replaced.

I did the test last night when the system is cold then you turn on the hot tap and the CH pipes did get hot, so yes I think the diverter valve has definitely gone.
Not sure if the diverter valve was cleaned during the last service but the plumber chap I'm dealing with has ordered a diverter valve service kit which fingers crossed will arrive today.
I'm still confused as to why the system is playing up when it's up to temperature and pressure then works fine when it's cold. Could it be that something in the diverter valve is sticking when it gets hot and then frees up when it cools? Or there something else gone?